Soft tissue pathology is a subspecialty that involves the diagnosis of lesions that are neither epithelial nor skeletal and do not originate from the central nervous system. This includes neoplasms of muscle, connective fascia, vasculature, and the peripheral nervous system [1]. With such an ambitious definition, the scope of this subspecialty is broad and encompasses a formidable number of entities with specific histologic, genetic, and clinical features.

When encountering such an abundance of terms and neoplasms, one can be tempted to view soft tissue tumors simply by histologic grade. Although this approach simplifies several hundred neoplasms into low-, intermediate-, or high-grade sarcoma, it overgeneralizes and is incompatible with increasingly individualized treatment regimens. By classifying soft tissue neoplasms to a particular histologic entity, pathologists have been able to attribute unique clinical behaviors to specific tumors and establish a conceptual framework for future advances in soft tissue oncology [2]. Consequently, clinicians who frequently treat sarcoma patients can benefit from understanding the diagnostic terms and approach of soft tissue pathologists.

In order to maintain consistency , the diagnostic nomenclature of soft tissue pathology is determined by a consensus of international experts. Every few years, this terminology is edited to reflect the addition of newly discovered neoplasms or reassignment of entities to different categories. These efforts are published as a volume in the World Health Organization Classification of Tumours book series, titled Pathology and Genetics of Tumours of Soft Tissue and Bone [3].

Unlike carcinomas that are often classified by site of occurrence (e.g. colorectal adenocarcinoma, gastric adenocarcinoma), a soft tissue tumor can arise in multiple areas and is not amenable to description by anatomic location. Instead, these neoplasms are described according to the connective tissue that they most closely resemble. Such entities include pleomorphic liposarcoma (Fig. 1.1) that resembles fat (Fig. 1.2) or leiomyosarcoma (Fig. 1.3) that resembles smooth muscle (Fig. 1.4). Many soft tissue tumors do not resemble any specific mesenchymal tissue and are simply designated as “tumors of uncertain differentiation” [3].

Following primary histologic classification, soft tissue tumors are further subdivided into clinical categories of benign, intermediate (locally aggressive), intermediate (rarely metastasizing), and malignant. Most benign lesions do not recur, and rare instances of recurrence are easily cured by complete local excision. Intermediate (locally aggressive) tumors will not metastasize but are prone to recurrence and require wide excision given their infiltrative growth pattern. Intermediate (rarely metastasizing) soft tissue tumors are typically locally aggressive, but demonstrate the capability to occasionally metastasize (less than 2% risk). Malignant tumors are locally destructive and often metastasize. Generally, high-grade tumors tend to metastasize more frequently than low-grade tumors [3].